The invention relates to the production of transparencies which may be utilized in visual presentations wherein images are commonly projected with magnification onto a screen for the simultaneous viewing by a plurality of observers. More particularly, the invention relates to a coating which can be applied to a transparent polymeric film base which is to be imaged by means of an electrostatic printing technique, such as xerography.
At present, xerographic copiers which use plain paper for reproduction of images from a master generally employ a dry powder, called "toner" to form the image. There is a special class of plain paper copiers, called "liquid toner copiers" which use a suspension of toner in a liquid, such as deodorized kerosene, but they are not included in the scope of this invention.
As is well known, transfer electrostatic copying commonly involves imparting a uniform electrostatic charge, either positive or negative, depending on the specific machine under consideration, to a photoconducting surface which will hold a charge only in the dark, such as a selenium coated drum. This may be accomplished by passing the drum under a series of corona-discharge wires in the dark. The photoconducting surface is then exposed through a lens system to a document or article bearing the image which is to be formed. In areas where light strikes the photoconducting surface the charge is dissipated and flows off through a conducting support to ground, with the electrostatic charge remaining largely intact in the image areas. Next, oppositely charged toner powder comprising a pigmented thermoplastic resin is brought into contact with the photoconducting surface and clings by electrostatic attraction to the charged areas of the surface. A sheet which is to receive the image is placed over the powder image, and is given a charge, such as by use of corona-discharge wires. As a result, a large portion of the charged powder on the photoconducting surface is transferred to the sheet. Finally, the toner is fused to the sheet by application of heat, pressure, or a combination of both.
The toner powder is generally comprised of a polymeric resin, a pigment, and various additives to control the surface charge and other characteristics necessary for reliable operation. The most common method of fusing the toner to the copy sheet is to heat it, either with lamps or heated rolls. Heated rolls combine heat and pressure, thus reducing the amount of heat needed. Cold pressure fusing, which uses no heat and very high pressures, may also be used.
When toner is fused to paper with cold pressure fusing, the crush rolls press the toner into the pores of the paper, with the result that mechanical, as well as adhesive, bonding occurs. The pores operate to increase the area available for adhesive bonding and to make the sheet more pliable to allow greater intimacy of contact. Moreover, because most of the toner ends up below the surface of the paper, it is protected from scratching by the structure of the paper itself.
When toner is fused to polymeric film with cold pressure fusing, fusing depends primarily on surface adhesion, with little or no mechanical anchoring and with no penetration of the film surface, because the film is not porous and is not as deformable as paper. The lack of deformability of the film surface is a major barrier to the intimate contact between toner and film, which is necessary for good adhesion.
There are at least three general approaches for fushing toner to a film. The first approach involves using coatings which provide adhesive compatibility with the toner resin, so as to form strong bonds between toner and film. An essential feature of any adhesion process is intimate contact between the surfaces to be adhered. If these surfaces are solid, a certain amount of conformability is needed to permit this contact. This first approach does not address this need for conformability. The second approach involves the creation of artificial pores in the film by blending an incompatible elastomer with a harder polymer and coating this mixture onto the film. The elastomer globules formed thereby appear as pores to the harder toner partices, thus providing a structure similar to that of paper. Although this second approach can provide good adhesion, it results in a very hazy film, on the order of 23 percent as measured in the Gardner hazemeter, compared to the 10 percent maximum usually desired for transparencies. The films produced by this approach tend to be sticky, with high friction, thus resulting in difficulty in feeding the copying machine. A third approach involves adding high levels of particulate material to the film coating, so as to create a rough surface on the film with many valleys and crevices, into which the toner can be pressed by the fusing process. If the coating resin also has high adhesive compatibility with the toner particles, the resulting film will have good toner adhesion. However, such a film would exhibit high haze.